1. Field of the Invention
The present invention relates to a light-guide sheet used mainly for operating electronic devices, and a movable contact unit and a switch employing the light-guide sheet.
2. Background Art
Recent electronic devices, typically mobile information terminals such as mobile phones, increasingly adopt light-emitting diodes or electroluminescent (EL) elements for lighting their control panels. This is to facilitate the identification and operation of buttons and display sheets, even in the dark. Movable contact units and switches that can be illuminated for easier visibility are also in demand for use in these devices.
A conventional movable contact unit and a switch are described next with reference to FIG. 11. FIG. 11 is a sectional view of the conventional switch. To facilitate understanding of the structure, dimensions are partially enlarged in FIG. 11. This switch includes movable contact unit 56, wiring board 57, light-emitting element 59, and display sheet 60.
Movable contact unit 56 includes light-guide sheet 53, film cover sheet 54, and substantially dome-shaped movable contacts 55 each made of a thin metal sheet. Light-guide sheet 53 includes light-transmissive film substrate 51, and luminescent protrusions 52 provided at predetermined points on the bottom face of substrate 51. Predetermined portions of the outer periphery of cover sheet 54 are attached to the bottom face of light-guide sheet 53 with adhesive (not illustrated). Movable contacts 55 are attached to the bottom face of cover sheet 54 under luminescent protrusions 52. Light-emitting element 59 such as a light-emitting diode is mounted on a top face of wiring board 57 at the right of light-guide sheet 53. The light-emitting face of light-emitting element 59 is faced toward a side face at the right end of light-guide sheet 53. FIG. 12 is a graph illustrating the relationship between optical wavelength and transmittance of light-guide sheet 53.
Wiring patterns (not illustrated) are formed on the top and bottom faces of wiring board 57. In particular, fixed contacts 58 are provided on the top face of wiring board 57. Each of fixed contacts 58 includes substantially round central fixed contact 58A and substantially U-shaped or ring-shaped outer fixed contact 58B around central fixed contact 58A. Movable contact unit 56 is attached to the top face of wiring board 57 such that the outer periphery of each movable contact 55 is placed on outer fixed contact 58B, and the center of the bottom face of movable contact 55 faces central fixed contact 58A with a predetermined distance in between.
Light-transmissive film display sheet 60 includes light-shielding portion 60A and display portions 60B. Display portions 60B are formed by cutting out the shape of the characters, symbols, etc., typically printed on the bottom face of display sheet 60. Display portions 60B are disposed over luminescent protrusions 52 of light-guide sheet 53.
The switch as configured above is placed on an operating face (not illustrated) of the electronic device. Central fixed contact 58A, outer fixed contact 58B, and light-emitting element 59 are connected to an electronic circuit (not illustrated) of the electronic device via wiring patterns.
In the above structure, when a user presses one of display portions 60B in display sheet 60, light-guide sheet 53 and cover sheet 54 dent, and a dome-like center of movable contact 55 is pressed. When a predetermined pressing force is applied to this center, movable contact 55 resiliently inverts downward with a click feeling, bringing the center of the bottom face of movable contact 55 down into contact with central fixed contact 58A. This contact electrically connects central fixed contact 58A and outer fixed contact 58B via movable contact 55.
When the pressing force on display sheet 60 is released, movable contact 55 resiliently reverts upward due to its resilient recovery force, separating the center of the bottom face of movable contact 55 from central fixed contact 58A. Accordingly, central fixed contact 58A and outer fixed contact 58B are electrically disconnected.
As described above, the device is switched to each function by electrical connection and disconnection of fixed contact 58. When power is supplied to light-emitting element 59 from the electronic circuit of the device, light-emitting element 59 emits light. This light enters light-guide sheet 53 from the side face at the right end, and propagates through leftward, causing a reflection in substrate 51.
The light is diffused and reflected in luminescent protrusions 52, and lights display portions 60B from beneath. Since display portions 60B are lighted, the user can identify the indications on display portions 60B, such as characters or symbols, even if the surrounding area is dark. The user can thus operate the device with ease.
For lighting display portions 60B by illuminating luminescent protrusions 52 of light-guide sheet 53, a white light is typically used as luminescent color of light-emitting element 59. White light is generally produced by mixing blue light and yellow light. As shown in FIG. 12, yellow light, which has a long wavelength, has high light-transmittance inside light-guide sheet 53, unlike blue light which has a short wavelength and low light-transmittance. Accordingly, if a small number of light-emitting elements 59, such as one or two, are used for lighting, luminescent protrusion 52 disposed near light-emitting element 59 produces white light of a mixture of blue and yellow. However, at luminescent protrusion 52 far from light-emitting element 59, blue light which has low transmittance weakens, and yellow light which has high transmittance is relatively intensified. As a result, the luminescent color of luminescent protrusion 52 at this point becomes yellowish. Illumination of display portions 60B thus results in uneven colors.
To prevent variations in luminescent colors of luminescent protrusions 52, and to uniformly illuminate display portions 60B, one way is to dispose numerous light-emitting elements 59 on the outer periphery of light-guide sheet 53. However, this results in a complicated and expensive structure.